The present invention relates to liquid crystal driving devices for driving matrix-type liquid crystal display panels having a tone display capability.
Conventionally, there have been proposed various techniques for reducing power consumption of liquid crystal display devices. For example, Japanese Publication for Unexamined Patent Application No. 281933/1997 (Tokukaihei 9-281933) (published on Oct. 31, 1997) discloses a technique in which a display memory is installed in a driver LSI of the liquid crystal panel to limit external access to the display memory when displaying a still image in particular, so that the power consumption of the liquid crystal display device can be reduced.
Such a liquid crystal display device is described below with reference to FIG. 10. It is assumed here that a liquid crystal panel 1003 is used in a matrix-type liquid crystal display device and is AC-driven according to a line-sequential scanning method and a voltage average method, both of which have been commonly used in this type of liquid crystal display.
The liquid crystal panel 1003 has such a structure that an STN liquid crystal element is placed between a pair of glass substrates, wherein, for example, a plurality of segment lines and a plurality of common lines are respectively disposed on the inner surfaces of the glass substrates orthogonal to each other, and each intersection of the segment lines and the common lines makes up a pixel.
Before explaining black-and-white display operations of the liquid crystal display device, an explanation will be given to a liquid crystal driver for use in liquid crystal display devices for portable phones and the like, in which a common driver, a segment driver, a display data memory, and a control circuit of the display data memory are packaged in one chip, for example.
A liquid crystal driver 1002 is provided therein a display data memory 1006, which corresponds one to one to the pixels of the liquid crystal panel 1003. For example, in order to carry out display in 248xc3x9768 dots, the display data memory 1006 requires a capacity of 248xc3x9768=16864 bits.
The liquid crystal driver 1002 outputs a segment signal that turns the display data memory 1006 to either 0 or 1 for ON/OFF of each pixel of the liquid crystal panel 1003. A line address counter 1009 selects data of one line of the liquid crystal panel 1003 from the display data memory 1006, and the segment signal is sent to the segment driver 1004 in the form of data so selected.
The segment driver 1004, in response to the segment signal, outputs a voltage for driving the liquid crystal panel 1003 according to the display data and feeds it to the segment of the liquid crystal panel 1003.
The display data is applied to the display data memory 1006 according to the data bus width of a CPU 1001, by incrementing the X address and Y address of an X address counter 1007 and a Y address counter 1008.
A common driver 1005 successively outputs line by line a scanning pulse that indicates an ON line of the liquid crystal panel 1003 and feeds it to the common of the liquid crystal panel 1003. ON pixels and OFF pixels of a line are decided by the voltages applied to the common and segment of the liquid crystal panel 1003. By successively driving the common lines, any characters or graphics can be displayed on the liquid crystal panel 1003.
As described, display is carried out on the liquid crystal panel 1003 by the display data memory 1006, the line address counter 1009, the segment driver 1004, and the common driver 1005. Therefore, no transfer of display data is required between the CPU 1001 and the liquid crystal driver 1002 so long as the display remains the same. Further, a change of display is accompanied by data transfer that can be separately carried out from the transfer of display data that is used for the display of the liquid crystal panel 1003. This allows for a slower data transfer speed and thereby reduces power consumption.
The display data memory 1006 corresponds to the pixels, one bit per pixel, so that two-level display of black and white is carried out.
Multi-tone display that displays intermediaries between black and white, rather than two levels of black and white, can be carried out by a frame rate control (FRC) method which realizes multi-tone display by varying the number of times each pixel of the liquid crystal panel 1003 is switched ON when displaying a single display data with a period of plural frames, or by a pulse modulation method in which a pulse width is varied in one frame to change ON time.
In order to carry out multi-tone display by these methods, the capacity of the display data memory 1006 needs to be increased to increase the number of bits per pixel of the memory, and the tone data of each pixel needs to be stored. For example, 3 bits are required for each pixel in display of 8 tones, and 6 bits for 64 tones and 8 bits for 256 tones.
Miniaturization of LSIs has advanced over the last years and it is now possible to increase the capacity of the display data memory 1006 to readily accommodate multi tones and multi colors.
However, increasing the number of display pixels and thus the number of common lines in the liquid crystal panel 1003 shortens the ON time of the liquid crystal pixels in one frame. Thus, when the liquid crystal panel 1003 is used to carry out large-screen or high-resolution display by the frame rate control method or pulse modulation method, flicker is caused in a display of tones with a short ON time and a poor display quality results.
In order to avoid such a problem, there has been proposed a driving method known as a dual-scan display mode, in which two segment drivers are used to simultaneously and respectively drive the upper and lower segments that have been prepared by dividing the display screen of the liquid crystal panel, i.e., the segment lines, into upper and lower parts.
Note that, a simple-matrix display mode in which the display screen of the liquid crystal panel is not divided into upper and lower parts will be called a single-scan display mode.
The dual-scan display mode, compared with the single-scan display mode, requires two driving circuits for the upper part and lower part of the liquid crystal panel, and therefore increases the scale of LSI mounting and the complexity of the driving circuits.
The advantage of the dual-scan display mode over the single-scan display mode, however, is that an ON time twice as long as that of the single-scan display mode (1/2 duty ratio) can be obtained. This prevents flicker on the display screen and improves display quality.
Meanwhile, in order to carry out multi-tone and multi-color display in the matrix-type STN liquid crystal display device while at the same time suppressing power consumption, a driver with a large display memory capacity needs to be used for the driving in the dual-scan display mode.
While such a liquid crystal display device for displaying a high-quality and multi-tone image is in demand, there has also been a demand for low-cost liquid crystal display devices which do not require many tones for display.
It is therefore advantageous, in terms of productivity and cost, to share the driver between these two different types of liquid crystal display devices.
However, an increase of display memory capacity is generally associated with an increase in unit cost of the driver, which increases production cost of the liquid crystal display device. Also, display panels with a small number of tones, because of their low unit sale price, cannot generally afford expensive drivers with a large memory capacity for displaying multi tones. It is therefore required to prepare and use a plurality of different types of drivers having different display memory capacities, according to different numbers of tones required for the display, depending on the use of the driver.
That is, two kinds of drivers are required: one for the liquid crystal display device that carries out high-quality and multi-tone display; and one for the liquid crystal display device that does not require many tones. This is disadvantageous in terms of reducing cost by mass-production. As a result, a high production cost is borne for the liquid crystal display device.
An object of the present invention is to provide liquid crystal driving devices that require low production cost for various types of liquid crystal display devices, by enabling a single driving IC to be shared between liquid crystal display devices that display high-quality and multi-tone images and liquid crystal display devices that do not require many tones.
In order to achieve the foregoing object, a liquid crystal driving device of the present invention is provided with a display memory which stores display data to be supplied to a matrix-type liquid crystal display panel having pixels that are disposed in row and column directions in a matrix for displaying 2k tones (k being a natural number), and a column driver of m outputs (m being a natural number) and a row driver of n outputs (n being a natural number), and the liquid crystal driving device includes: display memory control means for varying a value n and a value m of mxc3x97nxc3x97k bits, which is a capacity of the display memory, while holding nxc3x97k bits of mxc3x97nxc3x97k bits constant; and output number setting means for setting a number of outputs of the row driver such that the n value varied by the display memory control means becomes the number of outputs of the row driver.
According to the foregoing liquid crystal driving device, the display memory control means varies value n and value k to change the address of stored display data in the display memory of mxc3x97n bits. The change of value n and value k does not bring about a change of a capacity (mxc3x97nxc3x97k) of the display memory because nxc3x97k is held constant.
Varying value k changes the number of tones of the matrix-type liquid crystal display panel, and varying value n changes the number of outputs of the row driver.
Thus, by varying value n and value k, the number of outputs of the row driver can be set according to the number of tones, without changing the capacity of the display memory.
For example, the following considers a matrix-type liquid crystal display panel with 248 segment lines (lines in a column direction) and 64 common lines (lines in a row direction) that is dual-scanned to display 256=(28) tones. Here, k=8.
The dual-scan is a display mode in which two parts of a display screen of the matrix-type liquid crystal display panel, that has been divided in a row direction, are simultaneously driven.
In the dual-scan display mode, the number m of outputs of the column driver is 248, but the number n of outputs of the row driver is {fraction (64/2)}=32 since two liquid crystal driving devices are used. Further, since the number of tones is 256, k=8. Here, the capacity of the display memory is 248xc3x9732xc3x978=63488 bits.
Further, 248xc3x9732 is the number of pixels that are driven by a single liquid crystal driving device, and it indicates the count of addresses of the display memory that stores display data.
The following describes the case where the liquid crystal driving device adapted to the dual-scan is used for the single-scan.
The single-scan is a display mode in which the display screen of the matrix-type liquid crystal display panel is directly driven for display.
The single-scan display mode employs a single liquid crystal driving device and therefore the number of outputs of the column driver is 248 and the number of outputs of the row driver is 64. Here, the product of the number n of outputs of the row driver and the number k of tones is constant and k=4. Accordingly, the number of tones of the matrix-type liquid crystal display panel becomes 24=16. Here, the capacity of the display memory is 248xc3x9764xc3x974=63488 bits.
Further, 248xc3x9764 is the number of pixels that are driven by the single liquid crystal driving device, and it indicates the count of addresses of the display memory that stores display data.
That is, the dual-scan display mode and the single-scan display mode have different counts of addresses of the display memory that stores display data.
Despite these different display modes with different numbers of tones, the same liquid crystal driving device can be used in the dual-scan display mode as well as in the single-scan display mode by varying a value n and a value m of mxc3x97nxc3x97k bits, which is a capacity of the display memory, while holding nxc3x97k bits of mxc3x97nxc3x97k bits constant, and setting a number of outputs of the row driver such that the n value varied by the display memory control means becomes the number of outputs of the row driver.
By thus enabling the same liquid crystal driving device to be shared between different display modes with different numbers of tones, the effect of mass-production can be expected. That is, the cost per liquid crystal driving device can be reduced, thereby reducing the production cost of the liquid crystal display device.
Further, because the capacity of the display memory remains the same regardless of the number of tones, the cost of the liquid crystal driving device will not be increased by an increased memory capacity.
In order to achieve the foregoing object, another liquid crystal driving device of the present invention is provided with a display memory which stores display data to be supplied to a matrix-type liquid crystal display panel having pixels that are disposed in row and column directions in a matrix, and column and row drivers for driving the matrix-type liquid crystal display panel, and the liquid crystal driving device includes: setting means for setting a count of addresses of a display data storing area of the display memory, so as to set a number of outputs of the row driver according to the count, wherein the setting means varies the count of addresses between a dual-scan display mode in which two parts of a display screen of the matrix-type liquid crystal display panel, that has been divided into two parts in a row direction, are simultaneously driven to carry out display, and a single-scan display mode in which the display screen of the matrix-type liquid crystal display panel is directly driven to carry out display.
According to this liquid crystal driving device, the count of addresses of the display data storage area of the display memory is varied between the dual-scan display mode and the single-scan display mode, and the number of outputs of the row driver is changed according to this count. This enables the same liquid crystal driving device to be used between liquid crystal display devices of the single-scan display mode and liquid crystal display devices of the dual-scan display mode.
By thus enabling the same liquid crystal driving device to be shared between different display modes (dual-scan display mode and single-scan display mode) with different address counts of the display data storage area of the display memory, the effect of mass-production can be expected. That is, the cost per liquid crystal driving device can be reduced, thereby reducing the production cost of the liquid crystal display device.
For a fuller understanding of other objects, the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.